The present invention relates generally to antennas, and more particularly to a novel direction finding antenna of the type comprising two short spaced-apart monopole antennas top-connected by a transmission line.
In the field of antennas, it has been general practice to employ antennas which are narrow banded and exhibit widely variable impedance characteristics with respect to frequency. Further, antennas of similar appearance, exemplary of the prior art, often incorporate a terminating capacitor with one end grounded to the conducting surface, tending to limit the use of the antenna to a narrow range of radio frequencies, usually requiring adjustment of the capacitor for each separate frequency of operation.
It is known in the art to employ a U-shaped bar antenna, referred to as a "towel bar" antenna, as a homing device for aircraft. A single such antenna, marketed by Dorne and Margolin, Inc., is mounted on an aircraft, transverse to the direction of movement or flight path of the aircraft. Leads are connected from both ends of the antenna to a receiver and a signal processing circuit designed to terminate one end and feed the other end, and then reverse the connections, so that the directivity of the antenna is repeatedly switched 180.degree.. The fuselage of the aircraft serves as ground plane for the antenna.
In operation, the signal processing circuit for the system converts a signal received from a radio transmitter into alternate left- and right-looking antenna patterns. If the vehicle veers to the right or left of the direction to the transmitting source, this deviation is detected by the processing circuit and the appropriate error is displayed on a meter which is part of the circuit.
The problem with a U-shaped bar antenna, consisting of an open transmission line above a ground plane, is that the horizontal transmission line member of the antenna has to be a very specific, and therefore limited, height from the ground plane in order for the antenna output to be a practical impedance, necessary in matching the antenna to the receiver circuit. In the case of the Dorne and Margolin, Inc. "towel bar" antenna, the height from the ground plane was about 6 inches. A matching transformer was required at the antenna output, in order to match its impedance to the associated receiver. Such limited height adversely affects the signal pickup capability of the antenna. Even under the best of conditions, radio direction finding is subject to error. Things such as vehicle shape, propeller interference, other antennas mounted on the vehicle, all contribute to reading error. Every antenna pattern also changes shape as the frequency changes, further reducing reading accuracy. The worst condition is with mobile land direction finding. Very sizable errors are produced by the addition and cancellation of radio waves due to reflections from buildings, electric lines, roadways, cars, and trees as the vehicle moves along.
In prior application Ser. No. 758,591, there is disclosed a direction finding antenna system comprising a plurality of U-shaped antennas, each consisting essentially of three electrically active elements. One of the elements is an open transmission line spaced above and parallel to a ground plane, the other two being spaced-apart monopoles top-connected by the open transmission line. One of the monopoles preferably is connected to the ground plane by a terminating resistor, defining the antenna terminated end, the other monopole being connected to a receiver through an impedance transformer, defining the antenna output end. The plurality of the antennas are radially positioned about a central point with the output ends outboard of the terminated ends and uniformly spaced from each other.
In operation, a radio signal from a remote source impinges first on the monopole of an antenna closest to that signal, and induces a current in the monopole which is then conducted by the transmission line to the second monopole of that antenna. By virtue of this conduction, and the fact that the monopoles are top-connected, a current exists in the second monopole which is out of phase with respect to a current induced in the second monopole by the signal directly received. The two currents cancel each other. If the signal is to the right or left of the antenna alignment, the cancellation of currents is less than complete. By employing an array of antennas as described and a receiving circuit adapted to resolve the outputs of the multiple antennas, the exact direction toward the signal is determined.